Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, they are also used for in other areas, such as chemical vapor detection. Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the propagation path affect the characteristics of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the acoustic wave sensor.
Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure. An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. Bulk acoustic wave device are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of masks.
Acoustic wave devices, such as, for example, a surface acoustic wave resonator (SAW-R), a surface acoustic wave delay line (SAW-DL) device, a surface transverse wave (STW) device, or a bulk acoustic wave (BAW) device, have been utilized in mechanical quantities measurement. In such sensing applications, the sensing devices or components are typically clamped or oriented in the most sensitive direction to the mechanical quantities.
The most important difference between an acoustic wave device and a conventional wired sensor is that the acoustic wave device can store energy mechanically. Once such a device is supplied with a certain amount of energy (e.g., through RF—Radio Frequency), the device can operate for a time without any active parts (i.e., without a power supply or oscillators). Such a configuration makes it possible for acoustic waves to function in RF powered passive and wireless sensing applications.
One area where acoustic wave devices seem to have promise is in the area of wireless acceleration detection. One of the problems with current acoustic wave sensors utilized for acceleration detection is that such devices are limited both in their ability to accurately detect acceleration and their placement with respect to an accelerating body. This is because such devices are wired-based. To date, passive and wireless acoustic wave accelerometers have not been successfully implemented. It is believed that the device disclosed herein overcomes the problems associated with current acoustic wave sensing devices.